1. Field of the Invention
The present invention is related to an image processing method by adjusting heating time, and more particularly, to an image processing method applied on a thermal printer by adjusting heating time.
2. Description of the Prior Art
With the rapid development of digital cameras, the amount of digital images has increased tremendously. Though digital images can be viewed on display devices such as computer screens or televisions, so far these display devices have not been able to replace the need for paper photos completely. It is because people have become accustomed to enjoying images in the form of paper photos, and the feeling of holding photos and the pleasure of sharing them with someone around you can not be changed in a short time. Therefore, many printer manufacturers focus on printers with photo printing functions. In the consumer market, the demand for photo printers that specialize in digital image printing also increases as higher quality photo printing is more often required these days.
Based on different printing technologies, photo printers can be categorized into three main types: the laser printer, the inkjet printer and the thermal printer. Although color laser printers are available widely in the consumer market, laser printers are usually not a popular choice for photo printers due to higher prices and worse color expression in photo printing. The developments on photo printers are mainly focused on the inkjet printer and the thermal printer. An inkjet printer is a half-tone printing device that uses the dithering technique to place extremely small droplets of ink onto the paper to create an image. The purpose of the dithering technique is to create photo-quality images with these tiny dots as close to those seen by bare human eyes as possible. The half-tone technology cannot match the continuous-tone technology in printing effects. One can clearly distinguish the difference in printing quality between a half-tone photo printing and a continuous-tone photo printing, especially when the photos are being enlarged.
A thermal printer is a continuous-tone printing device that drives its thermal print head (TPH) based on image signals. When the temperature of the TPH is increased, it heats ribbons containing dyes and then diffuses the dyes onto specially coated paper or transparencies. Since the temperature of the TPH controls the amount of dye being transferred, a thermal dye transfer printer can express more color degrees and produce continuous-tone images that mimic actual photographs. Compared to other printing techniques, the thermal printer can produce continuous-tone and lifelike color images that best match the traditional paper photos. Due to its excellent printing quality and the natural, continuous color expression, the thermal printer is particularly suitable for photo printing applications.
Before printing digital images, image processing is usually necessary to achieve different printing effects. In the analog technology of a thermal printer, the heating time of the TPH is decided by the strength of the image signals and the amount of dye being transferred is controlled by the temperature of the TPH. To achieve the color density required by each color degree, usually several predetermined heating tables are provided to control the heating time of the TPH in accordance with each color degree, as illustrated in FIG. 1 through FIG. 3:
FIG. 1 shows the relationship between the color density D(t) and the heating time t.
FIG. 2 shows the relationship between the color degree T(x) and the heating time t:tx=T(x), x=0-255
FIG. 3 shows the relationship between the color density D(x)and the color degree x based on FIG. 1 and FIG. 2:dx=D(tx)=D(T(x)), x=0-255;
where x represents the color degree;
dx represents the color density each color degree x corresponds to; and
tx represents the amount of heating time required to achieve the color degree x.
In the prior art image processing method, a predetermined heating table tx is first decided, and then a function dx is obtained, showing the relationship between the color density and the color degree. During the image processing procedures, the color degrees of different printing parameters are adjusted respectively to achieve the required color density according to each printing parameter.
The following formulae illustrate a prior art image processing method for a printing parameter brightness:B(x)=x+Δx, when 0≦(x+Δx)≦255;  formula 1:B(x)=255, when (x+Δx)>255;  formula 2:B(x)=0, when (x+Δx)<0;  formula 3:
where x is original color degree, x=0-255;
Δx is the amount of brightness adjustment; and
B(x) is the color degree of the parameter brightness after brightness adjustment.
In the prior art image processing method for brightness, the color degree of the brightness parameter is being adjusted and Δx represents the amount of brightness adjustment. FIG. 4 shows the relationship between the color degree B(x) and the original color degree x after brightness adjustment (Δx>0). FIG. 5 shows the relationship between the color degree B(x) and the original color degree x after brightness adjustment (Δx<0). After brightness adjustment in the prior art method, some color degrees correspond to the same values of B(x). When x is greater than a certain value, the corresponding B(x) values are all 255, as illustrated in FIG. 4 and formula 2; when x is smaller than a certain value, the corresponding B(x) values are all 0, as illustrated in FIG. 5 and formula 3. These result in indistinguishable color degrees after brightness adjustment. Based on FIG. 3 and FIG. 4, FIG. 6 shows the corresponding relationship between the color density and the color degree after brightness adjustment (Δx>0). Based on FIG. 3 and FIG. 5, FIG. 7 shows the corresponding relationship between the color density and the color degree after brightness adjustment (Δx<0). In the prior art image processing method for brightness, the color degree of the brightness is being adjusted. Some color degrees which have different x values during brightness adjustment can correspond to the same values of B(x) and color density after brightness adjustment. Therefore some color degrees are indistinguishable in the prior art image processing method for brightness adjustment.
The following formulae illustrate a prior art image processing method for a printing parameter contrast:C(x)=Int(x−128)*R+128, when 0≦(x−128)*R+128≦255;  formula 4:C(x)=255, when (x−128)*R+128>255;  formula 5:C(x)=0, when (x−128)*R+128<0;  formula 6:
where x represents the original color degree, x=0-255;
R is a parameter for contrast adjustment; and
C(x) represents the color degree of the parameter contrast after contrast adjustment.
In the prior art image processing method for contrast, the color degree of the contrast parameter is being adjusted and R is a parameter for contrast adjustment. FIG. 8 shows the relationship between the color degree C(x) and the original color degree x after contrast adjustment (R>0). FIG. 9 shows the relationship between the color degree C(x) and the original color degree x after contrast adjustment (R<0). After contrast adjustment in the prior art method, some color degrees correspond to the same values of C(x). When (x−128)*R+128 is greater than a certain value, the corresponding C(x) values are all 255; when (x−128)*R+128 is smaller than a certain value, the corresponding C(x) values are all 0, as illustrated in FIG. 8, formula 5 and formula 6. These result in indistinguishable color degrees after contrast adjustment. Based on FIG. 3 and FIG. 8, FIG. 10 shows the corresponding relationship between the color density and the color degree after contrast adjustment (R>0). Based on FIG. 3 and FIG. 9, FIG. 11 shows the corresponding relationship between the color density and the color degree after contrast adjustment (R<0). In the prior art image processing method for contrast, the color degree of the contrast parameter is being adjusted. Some color degrees which have different x values during contrast adjustment can correspond to the same values of C(x) and color density after contrast adjustment. Therefore some color degrees can be indistinguishable in the prior art image processing method for contrast adjustment.
In the prior art image processing method, the color degrees of each printing parameters are adjusted. This method inevitably results in indistinguishable color degrees, which are undesirable during imaging processing procedures.